In recent years, fuel cells obtaining electric power from an electrochemical reaction of hydrogen and oxygen are studied for a variety of applications such as portable equipment, automobiles and the like. A fuel cell has a structure normally formed of several tens to several hundreds of unit cells stacked in layers in series. A unit cell is a basic configuration unit formed of electrolyte membrane, an electrode and a bipolar plate. Generally, a fuel cell is fabricated in a method, as follows: The electrolyte membrane and the electrode are previously formed as a membrane electrode assembly (MEA) and thereat the bipolar plate is disposed. The bipolar plate has channels formed at least one surface thereof for supplying hydrogen or a similar fuel, an oxidant formed of air or oxygen, and a coolant cooling the cells, respectively.
The bipolar plate is required to have sufficient conductivity to ensure electrical connection to an MEA adjacent thereto to allow the fuel cell to generate electric power more efficiently, and in addition thereto, it is also required to have sufficient mechanical strength to support the structure formed of unit cells stacked in layers. Furthermore, as there is a demand for fuel cells reduced in size, there is also a demand for bipolar plates reduced in thickness. Furthermore, there is also a demand for higher precision in thickness in order to reduce contact resistance between the unit cells stacked in layers.
Conventional fuel cell bipolar plates are formed of a material including resin and a carbon material, and introduced into a compression mold and pressurized and thus molded. Such fuel cell bipolar plates are formed in molds having a variety of structures, as conventionally proposed (see patent documents 1-5 for example.).
The compression mold is required to be capable of (1) exhausting efficiently and externally the air present in the mold and that present in the material molded and (2) discharging from the mold externally the material to be molded that is excessively introduced into the mold.
Generally, conventional compression molds are configured of a recessed mold half 101 having a recess (a cavity) 101a, and a projected mold half 102 having a projection (a core) 102a, as shown in FIG. 24A, and furthermore, to satisfy items (1), (2) above, recess 101a and projection 102a have sidewalls, respectively, opposite to each other to provide a share edge (a region P in the figure).
When such a mold is employed to mold a material to be molded 120a, the material that is excessive flows out of recess 101a and is discharged into a gap (or clearance) of the share edge and the mold and the material can also have their internal air exhausted out of the mold efficiently.
Furthermore, there is also a mold, as shown in FIG. 25A, which is a joined-type compression mold configured of mold half 101 having recess 101a and mold half 102 having recess 102a. This type of mold is simple in structure and can also be reduced in thickness in total.
Patent Document 1: Japanese Patent Laying-open No. 2001-198921
Patent Document 2: Japanese Patent Laying-open No. 2003-170459
Patent Document 3: Japanese Patent Laying-open No. 2004-230788
Patent Document 4: Japanese Patent Laying-open No. 2004-71334
Patent Document 5: Japanese Patent No. 3751911